During a process of manufacturing a glass optical fiber, a glass fiber is drawn from a preform and then coated with one or more coating materials. The coating materials include, for example, polymeric compositions and are applied by one or more coating applicators. The function of the fiber coating is to protect the surface of the glass optical fiber from mechanical scratches and abrasions which the optical fiber may experience during subsequent handling and use. The coating or coatings also influence the optical fiber's optical characteristics in response to external mechanical forces and environmental temperature.
Polymeric coatings are effective to prevent mechanical damage to the glass fiber surface they are meant to protect, however, diffusion of water vapor, hydroxyl ions, and hydrogen through the polymeric coatings pose additional threats to the strength, mechanical integrity and optical performance of the optical fiber.
Mechanical failure of the optical fiber may occur through a glass fiber failure mechanism referred to as stress corrosion. In an outer surface of a glass body, there exist surface imperfections resulting from mechanical damage or flawed silica bonds, for example. These imperfections, which are called microcracks, act as stress concentrators and thus may cause failure to occur preferentially at these locations when the fiber is subjected to tensile stresses. As stress is increased to a certain critical level the fiber will fail catastrophically at the crack site. Normally, these cracks will not grow under the influence of stress alone. In the presence of contaminates, hydroxyl ions, for example (the source of which may be water vapor), these cracks tend to grow at predictable rates when subjected to tensile loading. This stress corrosion is the result of the incorporation of the hydroxyl ions into the silica matrix of the optical fiber. Fiber failure may occur at stress levels significantly below an otherwise higher level due to the fact that the microcracks slowly but steadily reduce the area over which the tensile loads are resisted.
The presence of hydrogen adjacent to the optical fiber can also result in the diffusion of hydrogen through the polymer coatings and into the fiber core. Hydrogen which has diffused into the core of the fiber may react with core glass matrix defects, the effect of which is increased optical loss in the fiber.
Stress corrosion and hydrogen absorption can be prevented or at least reduced to a significant degree by the application of a hermetic coating to the fiber surface after fiber drawing but before any polymeric coatings are applied. Hermetic coatings include metallic or non-metallic coatings applied to the fiber surface by a variety of methods. For example, J. A. Wysocki U.S. Pat. No. 4,407,561 discloses that a variety of metals, including nickel, copper and aluminum may be used to provide a hermetic coating for a glass optical fiber. The metallic coating is applied by passing a just-drawn optical fiber through a molten pool of metal.
Hermetic coating techniques are typically employed during the drawing of the glass optical fiber from its precursor preform. The hermetic coating, the source of which is the products of reaction resulting from a reaction between a reactive gas and heat of a heated optical fiber, is applied immediately after drawing because the hermetic coatings must be applied adjacent to the glass fiber surface prior to the deposition of any polymeric coatings. It would be impractical to take up bare, glass fiber and, then, later apply the hermetic and polymeric coatings.
Because the purpose of applying a hermetic coating is, in part, to preserve the strength of the fiber to which it is applied, it is necessary that the pristine glass surface that results from the fiber drawing operation is not damaged before either the hermetic or the polymeric coatings are applied. Therefore, the optical fiber must not make any mechanical contacts with any portions of a hermetic coating apparatus through which the fiber moves. Any orifice portions the fiber is caused to move through must be sized and oriented such that fiber mechanical contact with those portions is avoided.
Typically, an entire preform will be drawn in a single draw episode. That episode will be shorter or longer depending on the size of the preform. With larger preforms come longer episode periods. During optical fiber drawing, internal portions of the hermetic coating apparatus are exposed to the heat of the fiber and become heated. These portions become hotter as the fiber drawing episode continues. They may become so hot that spontaneous reaction of the reactive gas occurs at the surface of these portions. The reaction causes products of reaction to be deposited on these surfaces and over time these products will accumulate and form what is referred to as secondary deposition deposits or soot.
Another potential source of soot is the reaction of the reactive gas that occurs away from the fiber outer surface yet within a reaction chamber of the hermetic coating apparatus. The resulting soot may remain suspended in the reactive gas flowing through the reaction chamber of the hermetic coating apparatus and then be expelled through a reaction chamber exhaust outlet or the soot may become attached to internal portions of the hermetic coating apparatus.
Soot deposits are particularly troublesome when the site of the deposition and accumulation occurs at fiber entry or exit orifices. Because these orifices are quite small in diameter, attenuation of the orifice diameter can occur quickly. When the effective orifice diameter becomes so small as to interfere with the fiber, fiber surface abrasion occurs.
What is needed and what does not appear to be provided in the prior art are methods of and apparatus for preventing the accumulation of reacted material on portions of a hermetic coating apparatus. Generally, the sought-after design should be one that does not compromise the integrity of the hermetic coating apparatus, is easy to use and allows for extended draw run periods.